function xp = sistema(t, x)

a= 2e-6;
b= 1e-6;
r= 0.001;
s= 0.01;

xp(1) = r*x(1) - a*x(1)*x(2);
xp(2) = -s*x(2) + b*x(1)*x(2);

xp = xp';